Graceful error handling

[tlack]

Small suggestion:

When I enter some hamfisted bizarre totally wrong K/Q code (the only kind I know how to write) in the cells, the error text is splayed down the columns with one character of the error symbol per cell.

Though I know this is just a demo, it probably wouldn't be too hard to show errors in an alert() or an error messaging div. As an interactive way to learn the language, might be a handy addition.

[srpeck]

Not graceful, but now at least the error displays directly in the cell.